Epithelial cell adhesion molecule (EpCAM) is a 40 kDa transmembrane glycoprotein showing frequent overexpression in several human malignancies [Spizzo et al., 2004; Went P et al., 2006; Wenqi D et al, 2009]. EpCAM was originally identified as a cancer marker, attributable to its high expression on rapidly proliferating epithelial tumors [reviewed in Trzpis M et al., 2007]. The normal epithelia express EpCAM at a variable though generally lower level than cancer cells. It is also overexpressed in normal stem and progenitor cells [Stingl J et al., 2001; Schmelzer E et al., 2007; Trzpis M et al., 2008] and in cancer-initiating cells in breast, colon, pancreas and prostate carcinomas [Al-Hajj M et al., 2003; O'Brien C A et al., 2007; Ricci-Vitiani L et al., 2007]. Recently, EpCAM has been detected in circulating tumor cells expressing E6/E7-HPV oncogenes in peripheral blood in cervical cancer patients after radical hysterectomy [Weismann P et al., 2009]. There is a large database on EpCAM staining for many cancers and normal tissues. However, all these studies used antibodies directed against the extracellular domain of EpCAM that may detect the EpCAM precursor or cell-bound EpEx (the “extracellular domain”), or both [Wenqi D et al., 2009].
EpCAM is a pleiotropic molecule that serves important roles in cell adhesion, cell proliferation, differentiation, migration, cell cycle regulation and is implicated in cancer and stem cell signaling [Munz et al., 2009]. The molecular mechanisms that regulate EpCAM expression are not well understood. Recently, regulated intramembrane proteolysis (RIP) has been shown to act as its mitogenic signal transducer in vitro and in vivo [Maetzel et al., 2009]. The cleavage and shedding of EpCAM ectodomain, EpEx, by proteases-TACE and Presenilin-2, releases its intracellular domain (Ep-ICD) that translocates to the nucleus. The association of Ep-ICD with FHL2 and Wnt pathway components—β-catenin and Lef-1 forms a nuclear complex that binds DNA at Lef-1 consensus sites and induces gene transcription, leading to increased cell proliferation and has been shown to be oncogenic in immunodeficient mice [Maetzel, 2009]. In view of the multiple roles of EpCAM as an oncogenic signal transducer, cell adhesion molecule and cancer stem cell marker [Litvinov S V et al., 1997; Munz et al., 2009], it is important to establish the clinical significance of nuclear Ep-ICD in human cancers.
Nuclear Ep-ICD was recently reported in a preliminary study in human colon cancer, but not in the normal colonic epithelium [Maetzel, 2009]. In view of the tremendous heterogeneity in solid tumors, the clinical significance of nuclear Ep-ICD in other human cancers remains to be established. Further, EpCAM has been shown to increase cell proliferation by upregulation of c-myc, cyclins A and E [Munz et al., 2004].
Thyroid cancer (TC) represents 90% of all endocrine malignancies with an estimated annual incidence of 122,800 cases worldwide and approximately 33,000 newly diagnosed cases in the USA [Reis et al., 2005; Jemal et al., 2008]. Anaplastic thyroid cancer (ATC) is a rare but very aggressive form of this malignancy, accounting for less than 2% of all thyroid cancers. ATC commonly presents as a rapidly increasing neck mass that spreads locally, compresses the adjacent structures, with a tendency to disseminate to regional lymph nodes and distant sites [Pasieka J L et al., 2003; Are C & Shaha 2006]. Most well differentiated thyroid cancers have an excellent prognosis, with relative 5-year survival rates above 95%, despite their tendency for early metastasis. However, the less-differentiated thyroid tumors—anaplastic and other aggressive metastatic thyroid cancers can be fatal with median survival time ranging from 4 months to 5 years [Are C & Shaha, 2006]. This variation in clinical outcomes may be attributed to the differences in genetic damage acquired by the aggressive and non-aggressive thyroid tumors during their malignant evolution.
The pathogenesis of ATC is linked to mutations in BRAF, RAS, β-Catenin, PIK3CA, TP53, AXIN1, PTEN and APC genes [reviewed in Smallridge R C et al., 2009]. The gene expression signatures in ATC have been identified showing the upregulation of the serine/threonine kinase Polo-like kinase 1 (PLK1) and its potential as a therapeutic target in ATC has been investigated [Salvatore G et al., 2007 CR; Nappi T C et al., 2009]. However, there are no proven predictive molecular markers to identify aggressive TCs.
Further, well-differentiated papillary thyroid carcinoma (“PTC”) generally has a good prognosis and can be effectively managed through a combination of surgery and radioactive iodine treatment. Yet, a subset of PTC show poor prognosis and tumor recurrence may lead to increased mortality [Lin J D et. al., 2009]. The lack of universally accepted biomarkers to define aggressive PTC further confounds the difficulty in determining which PTC patients have a poor prognosis and those who do not. According to American Thyroid Association (ATA) guidelines, there are conflicting data outlining the patients to whom radioiodine remnant ablation should be given. This discrepancy might be caused by the limitation faced by the current system in differentiating between aggressive and non-aggressive PTC. A variety of factors have been studied and found to affect the prognosis for PTC patients including age, gender, tumor histology, extracapsular extension, tumor size and positive lymph nodes or distant metastases [Sipos J A et al., 2010]. Despite the plethora of criteria to differentiate aggressive PTCs from non-aggressive cases, lack of universal consensus and controversy generating ongoing debates resulted in only a select few being considered in the currently recommended TNM staging system [Sipos J A et al., 2010].
There is an urgent need to find reliable biomarker(s) that can aid in the identification of patients with aggressive PTC. The early stratification of patients with a poorer prognosis would enable oncologists to choose the treatment strategy that matches more closely to the patient's cancer biology, thereby avoiding over-treatment, improving survival and quality of life for benefiting patients, particularly in a population that has demonstrated an enhanced risk for aggressive tumors. Biomarker(s) that serve as a tool for timely intervention and detection could direct effective adjuvant treatment to those patients who require it and save patients with non-aggressive PTCs from unnecessary additional surgery and radiation. Subsequently, these novel marker(s) would possess the ability to pave the way for a revised, comprehensive, universally accepted management plan for PTC.